As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as a power source for the mobile devices. One of the secondary batteries is a lithium secondary battery having high energy density and discharge voltage, into which much research has been carried out and which is now commercially and widely used.
Based on kinds of external devices in which the secondary battery is used, the secondary battery may be used in the form of a single unit cell or in the form of a battery pack including a plurality of unit cells electrically connected with each other. For example, small-sized devices, such as a mobile phone, can be operated for a predetermined period of time from the output and the capacity of a single unit cell. On the other hand, large-sized devices, such as lap-top computers or electric vehicles, require use of a medium- or large-sized battery pack because a high output and a large capacity are necessary.
The battery pack is a battery structure including a plurality of unit cells electrically connected in series and/or parallel with each other. A wire, a plate, and a flexible printed circuit board (FPCB) are used in the battery pack so as to accomplish the electrical connection between electrodes of the unit cells.
The wire is a linear conductive member. Generally, an insulating resin is covered on the outer surface of the wire, and therefore, the wire has advantages in that the linear conductive member is easily deformed, and the costs of the liner conductive member are inexpensive. However, the wire has problems in that it is difficult to accomplish the electrical connection between the electrodes of the cells by a welding process, such as spot welding, ultrasonic welding, or laser welding, and a large amount of heat is transferred to the cells during the welding process, which causes damage to the cells.
The plate, which is a plate-shaped conductive member, has an advantage in that the electrical connection between the electrodes of the cells can be easily accomplished by the above-mentioned welding process. However, the plate has a problem in that it is difficult to accomplish the electrical connection between the electrodes of the cells even with only a slight-degree error.
The FPCB, which is widely used in recent years, has advantages in that the electrical connection between the electrodes of the cells can be easily accomplished like the plate, and it is suitable for the electrical connection in a complicated structure. However, the FPCB has problems in that the FPCB is very expensive, the formability is lowered, and the assembly operation is difficult.
Processes for constituting a battery module through the electrical connection between electrodes of a plurality of cells using an FPCB are partially shown in FIGS. 1 and 2. FIG. 1 illustrates a normal assembly process of the battery module in which the connection between the electrodes is normally accomplished, and FIG. 2 illustrates an abnormal assembly process of the battery module in which the connection between the electrodes is abnormally accomplished.
Referring to FIGS. 1 and 2, an FPCB 10 includes a flexible board 12, on which a circuit 14 is formed. To the circuit 14 are connected electrodes 22 of cells 20. When the connection circuit 14 of the FPCB 10 is properly arranged on the electrodes 22 of the cells 20, which are disposed regularly, as shown in FIG. 1, normal electrical connection between the electrodes is accomplished. When an error occurs at the first connection process as shown in FIG. 2, on the other hand, it is difficult to accomplish subsequent electrical connections between the electrodes. Consequently, the assembly operation is inaccurately carried out.
For a high-output, large-capacity battery module used in large-sized devices, such as electric vehicles and hybrid electric vehicles, high current flows during the charge and discharge of the battery module. Consequently, it is difficult to provide a desired output when the resistance at the electrically connected regions, for example, between the electrodes of the cells and the connector is high. According to circumstances, the electrode connector may be excessively heated, which considerably deteriorates the safety of the electrode connector.